Detection of respiratory waveforms using non-contact electrodes during bathing

The aim of this study is to develop a method for measuring the respiratory waveform using non-contact electrodes during bathing. To determine the most appropriate electrode arrangement, we modeled a composite system consisting of a body submerged in bath water. We calculated the frequency dependence of the impedance amplitude using a three-dimensional finite difference method (3D-FDM). The simulation results showed that an increase in chest size due to inspiration caused a decrease in the impedance amplitude in the frequency range of 0.1 Hz to 1 MHz. Next, bioelectric impedance (BEI) was measured in the frequency range of 4 kHz to 4 MHz at the maximum-end-expiration and maximum-end-inspiration stages. BEI results were consistent with those obtained from the model simulations. We found that 1 MHz was the appropriate frequency for measuring the respiratory waveform, and the time dependence of the impedance amplitude was measured at 1 MHz. The impedance amplitude agreed well with the respiratory waveform obtained from rubber strain gauge plethysmography, which was used as a reference.